The human anatomy includes many types of tissues that can either voluntarily or involuntarily, perform certain functions. After disease, injury, or natural defects, certain tissues may no longer operate within general anatomical norms. For example, after disease, injury, time, or combinations thereof, the heart muscle may begin to experience certain failures or deficiencies. Certain failures or deficiencies can be corrected or treated with implantable medical devices (IMDs), such as implantable pacemakers, implantable cardioverter defibrillator (ICD) devices, cardiac resynchronization therapy defibrillator devices, or combinations thereof. The electrical therapy produced by an IMD may include, for example, pacing pulses, cardioverting pulses, and/or defibrillator pulses to reverse arrhythmias (e.g. tachycardias and bradycardias) or to stimulate the contraction of cardiac tissue (e.g. cardiac pacing) to return the heart to its normal sinus rhythm.
In general, the IMDs include a battery and electronic circuitry, such as a pulse generator and/or a processor module, that are hermetically sealed within a housing (generally referred to as the “can”). An implantable lead interconnects the IMD and the heart. Typically, a medical electrical lead includes a flexible elongated body with one or more insulated elongated conductors. Each conductor electrically couples a sensing and/or a stimulation electrode of the lead to the electronic circuitry through a connector module. Electrical signals are transmitted between the electrodes and the pulse generator. For an IMD, functional implant life time is, in part, determined by the energy delivered per pulse. The IMD will have a longer life if the energy delivered per pulse can be maintained at a minimum. Designs of the lead and of the electrodes which are used with the lead are influenced by the electrical signal required for pacing stimulation. Physiologically, the IMD should be capable of generating a signal with a sufficient magnitude to depolarize the excitable cells of the myocardium to initiate contraction. The electrode shape, size, surface area, material and impedance combine to determine the energy required of the IMD.
In the context of medical electrical leads, a tubular electrode may typically be mounted around the exterior of an insulative lead body and coupled to an elongated conductive coil within the lead body. Different combinations of materials have been proposed for the electrode and conductive coils in the lead construction. However, the inventors of the present disclosure have found that conventional techniques utilized in joining different combinations of materials present challenges in the construction of leads having different combinations of materials. For example, the techniques utilized in joining some of these materials have been found to result in formation of intermettalics when the materials used have incompatible compounds. A property of intermettalics is brittleness which results in cracks and uneven surfaces thereby compromising the electrical conductivity and mechanical integrity of the lead.
Some proposals to overcome the above and other disadvantages have included cladding the conductive coil with a suitable material prior to coupling with another component. For example, the conductive coil may be cladded with the suitable material and the cladded portion of the conductor is then welded to the other component.
Therefore, there remains a need for an improved method of constructing an implantable lead having incompatible materials that are coupled directly, while maintaining the desired electrical conductivity and mechanical integrity.